In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility as in the case of polysilicon or microcrystalline silicon and having uniform element characteristics as in the case of amorphous silicon. A metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Insulated-gate field-effect transistors (hereinafter simply referred to as transistors) in each of which such metal oxide having semiconductor characteristics is used for a channel formation region have already been known.
It is known that in a transistor including an oxide semiconductor, degradation of characteristics, such as shifts in the threshold voltage and increase in off-state current, is caused by light, particularly light in the range of the wavelength shorter than that of the visible light, such as ultraviolet light. A transistor used in a semiconductor device desirably has small change in characteristics or small variation caused due to change in characteristics. Therefore, a technique for preventing degradation of characteristics of a transistor, caused by light, has been researched and developed, as disclosed in Patent Document 1 and Patent Document 2.